Automatic telephone system



1' 254 I Aprll 19,1927. T G. MARTIN .8

AUTOMATIC TELEPHONE SYSTEM Original Filed March 15 1921 Talb'cTI'E. I

Patented Apr. 19, 1927.

rarer TALBOT G. MARTIN, (333 CHICAGO, ILLTIIQIS, ASSIGIIOR, BY MEFSIIE ASSIGNMENTS, TO

AUTOMATIC INC, OF CHICAGO, ILLINOIS, A CORPORATION OF DELA- WSAI-LE.

Original application filed. March 15, 1921, Serial No. 452,468, new Patent No. 1,539,132, dated may 26,

The present invention relates in general to automatic telephone systems, but more especially to large automatic systems which employ both primary and secondary line switches for the purpose of connecting calling lines with idle first selector switches; and the object of the invention is the provision of various improvements in system of this kind which tend to increase its utility and commercial success. In particular the invention contemplates the use of secondary line switches of the rotary type, and arrangements are shown whereby such switches, may be used to advantage in conjunction with primary line switches of the well known Keith or plunger type. li-iany of the features, however, are not limited'to this arrangement.

A special object is the provision of novel and improved auxiliary circuits for a pri mary and secondary line switch system. These circuits have to do particularly with an arrangement for making a group of secondary switches busy to primary switches in case all trunks accessible to such second ary switch become busy, or in case a fuse supplying battery current to such secondary switches should blow. Arrangement are also provided for picking up the plungers of the primary switch-es under these circumstances and for testing the pickup circuit in case it should get out of order. In connection with the primaryline switches means is provided for preventing the operation of any switch of a group in case all the trunks accessible to .such group are busy and for signalling an operator if a subscriber calls while his line switch is inoperative.

This application is a division of myprior application, Ser. No. 52,468, tiled March 15, 1921, patented May 26, 1925, No. 1,539,132, in which the above features are described and illustrated but not claimed.

The foregoing features and others not now specifically mentioned will be described hereinafter with reference to the accompanying drawings, which show suliicient ot the apparatus in an automatic telephone system to enable the invention to be explained and understood.

Referring to the drawing, the reference character A indicates an automatic telephone substation ot the ordinary type hav- Divitled and this application filed April 6, 1925.

'acter E.

I derstood.

Serial No. 20,885.

ing the usual talking instrumentalities and a calling device S for controlling the automatic switches the exchange. The line conductors 11 and 12 terminate at the exchange in the individual line switchC, which is a line switch of the well known Keith or plunger type, and hence will not need to be described in detail. Briefly stated, the function of the line switch C is to extend the associated line when calling to an idle trunk line. In accordance with the usual practice, the lines entering the exchange are divided into groups, and all of the line switches as sociated' with the lines in each group may be controlled in their trunk selecting operations by a common master witch. The master switch which is associated with the group which includes the line switch C is shown in the lower left-hand corner of the sheet and is indicated by the reference char- This master switch is of the well known solenoid type such as s in common use in the automatic telephone exchanges throughout the country.

Secondary line switches are employed in the present system and accordingly the trunk lines from the primary line switch-boards will extend to secondary line switches rather than to first selector switches. The arrangement of the trunking between the primary and secondary line switch-boards is well un- There may be a plurality of groups of secondary line switches and each group of primary line switches will have outgoing trunks extending to secondary line switches in each secondary group, so that any calling subscriber will be given access to the trunk lines outgoing from all of the secondary groups. In this manner a large number of first selectors, say one hundred or more, may be combined in one large group thus securing an increased trunking efficiency. The admntageg gained by the use of secondary line switches, however, are well known. and need not be discussed in detail.

One of the trunk lines outgoing from the primary group which includes the line switch 0 is shown in the drawing, and extends to the secondary line switch D. The line switch D is of the well known rotary type, having wipers which normally stand on the trunk line last used, and which move in a forward direction only. As usually ill constructed, this line switch has a seniicircular bank comprising; twentydire contact set and double wipers standing at an angle f 180 degrees apart. V

The jack (l and the lamp ll are provided for a particular purpose to be explained hereinafter and are located at the wire chiet s, or complaint opcrator s desk. This jack is individual to the particular primary group which includes the line switch (l. and there may he a similar jack tor each ot the other primary groups. ".lfhere are a number of relays at the rightol the drawing: which have certain functions to perform when all of the trunk lines outgoingfrom the sec-- ondary line switch group which includes the line switch I) become busy, and at other times. It will be convenient, however, to go into this later on during the explanation of the operation of the system.

Having described the ap iniratus inrolved briefly. its operation will now be described more in detail with a view to pointing out the particular features oli novelty disclosed herein. For this purpose it will be assumed that the subscriber at substation A. desires to call some other subscriber in the system, and the operation of the primary and sec ondary line switches C and D in connecting the line of substation A to a trunltline extending to a first selector switch will be described. lVhcn the receiver is removed at substation A a circuit is completed over the line conductors 11 and 12 for the line relay 16 of the line switch (.l. Upon energizing, the line relay 16 closes a circuit for the pull down winding 18 of the line switch at armature 25. Upon energizing, the pull down winding operates both the cut-oil armature 524i and the plunger arm 23. the former being: ell'ective to disconnect the line conductors 11 and 12 troni the winding of the line relay and from the grounded connnon conductor 4-6, respectively; while the latter Forces the bank springs 30 31. 3-2, and 33 into engage nient with contacts 34-. 35, 36, and 3?, respectively. By the engagement ot bank springs 30 and 33 with contacts 3st and 3?. the line conductors ii and 12 are extended into connection with the trunk line conductors all) and 4.2. and a circuit is accordingly completed tor the line relay 5t) oi the secondary line switch D. The line relay 50 is accordingly energized and at its arn'iature 5Z3 closes a l'iolding circuit For the line switch (l. which may be traced from ground by way of the working contact oil? armature 5 -3 and the said armature, normally closed contact springs controlled by arinature 58, release trunl; conductor l1, contact 85. bank spring 3i, and the holdingwinding: 17 of the line switch (l to battery. This holding circuit is established beiliore the slow acting; line relay 16 of the line switch has time to deenergize and is elilective to retain the plunger lessees arm 23 and the t-ut'oll armature 241 in op- Grated position. A ground potential is also placed on the private norinal conductor 15 which extends to multiple test contacts on. the banlca ol' the connectors: which have access to the line ol substation A. whereby nardctl against intrusion in the the line is customary nianucr. .\notl:cr circuit closed by the o ieiation o l' the line switch (1 complctcd by the cnga nuucnt ot banlc springljs with contact till. This places a groinid on conductor ll: and a circuit is thus coinplctci'l by way o l' the master switch test wiper ltlt and the lcl't-luuu'l winding ot the starting; relay too to battery. Relay is accordingly energized and at its lett-hand armature coiuplt-ites a circuit for the locking relay, or magnet .llll. As is well undcn'stotnl when the locking relay is energized it re leases the master switch shalt (controlling all the idle line switch plungcrs) and the shalt instantly turns in a counter clock-wise direction under control ot a governor, the opcratinr :l'orcc being supplied by a heavy spring under tension. The test wipers 10p and till are attached to this shaft, from which it will appear that these test wipers will iuniuu'liatcly be moved oil from their a:1- ociatcd tenth test contacts and into ongagement with the associated ninth test contacts. As uming that the ninth trunk line is idle. there will be no ground potential on the ninth test contacts, the starting relay lllt) will dccncrgize to open the circuit of the locking relay 10.1., and the latter relay will (lt t1ltl. l5(3 to again loci; the master switch shalt. It may be pointed out in this connection that when the locking; relay 101 is energized it closes a circuit for the open main relay 10-; which disconnects the common coi'uluctor it from the nongrounded pole of the battery. This is to prevent the operation of any other line switches while the master switch shalt is being moved from one trunk to another by the master switch. Continuing now with the operation ot the secondaiy lino switch l), when the line re lay 50 is energized as cx daintu'l hereinbcto c, it closes a circuit at its arn'niturc til; which includes the winding of the switching relay 51 and the winding of the stepping magnet 52 in series, while at the same time by means of arinatiure the test wiper (it; connected to this circuit at a point between the said relay and the said magnet. The operation now depend: upon whether the trunk line with which the line switch wipers are associated is busy or idle. 1t this trunk line is busy there will be a ground pi. tcntial on the test contact: engaged by test; \Vll iti till. such as test contact tit) for example, the switching relay 51 will be short cirended. and the stepping! inagnet being: supplied with direct ground via the test wiper will operate in the manner ol a buazer lit) ill)

lllfi to advance the switch wipers step by step in search of an idle trunk line. This movement will continue as long as the test wiper (5(3 continues to engage grounded test contact-s. Then the wipers arrive at an idle trunk line, which we may assume is the trunk line comprising conductors 77, T8, and 79, the test wiper 66 will find no ground potential on the test contact 72, the rotation of the switch will cease and the switching relay 51 being no longer short circuited will energize in series with the stepping magnet 52, the latter remaining inoperative on account of the high resistance of the switching relay. Upon energizing, relay 51 connects the incoming release trunk conductor all to the test wiper 66 by means of armature 58, and at arma: ture 57 connects its own lower terminal to the now continuous conductor extending from the test wiper 66 back over conductor ell to the line switch C. The line relay 50 being still energized, the test wiper (i6 is connected to ground by these operations in order to render the selected trunk line busy. In ad dition to the foregoing, at armaturcs 56 and 59 relay 51 disconnects the incon'iing trunk conductors etO and 42 from the winding of the relay 50 and from ground, respectively, and extends them by way of 'ipers 65 and (3?, bank contacts 71 and 73 and conductors 77 and 79 to the line relay of the first selector switch in which the selected trunk line terminates. Although this switch is not. shown, its ope 'ation is well understood and it will be sutlicient to state that the line relay of the selector is energized over the circuit of the calling line and closes a circuit for the associated slow acting release relay. The latter relay, upon energizing, places a ground potential upon the release trunk conductor 78, thereby establishing a holding circuit for the switching relay 51 of the secondary line switch D, and for the holding winding 1? of the primary line switch C. This holding circuit, it will be understood, is established before the slow acting line relay 50 has had time to deencrgize.

The remainder of the operations in which the calling subscriber controls the first selector taken for use, and a plu 'ality of other automatic switches through the medium (it his calling device S, to complete the connection to the desired called line, are well understood and as they have nothing special to do with the present invention. these operations will not be vt'urther considered. ex-

cept to say that when the calling subscriber has finished talking, the restoration of his receiver to the switch-hook causes the automatic switches to restore to normal with the result that ground is disconnected from the release trunk conductor 78. This operation breaks the holding circuit for the primary and secondary line switches C and D and-the said switches are restored to normal position,

it being'understood that the restoring opera-- tion at the secondary line switch D consists merely in the dcenergization of the switching relay 51.

Having described the system in a general way, it will l e convenient now to explain the various features which i have incorporated therein with a View to enhancing its commercial success. One of these features is a new arrangement for making all of the trunk lines incoming to a group of secondary line switches busy whenever all of the trunk lines outgoing "from such secondary group become busy. it will be agprcciatcd that it is necessary to do this, since it calls were permitted to arrive at a roup of secondary line s itches when no trunks were available;

outgoing from such group. it would inevitably happen that surh calls would be lost, i. e., they could not be successfully completed. In order to accomplish the desired result. and considering the group or second ary line switches which include the line swrch D, a series of chain relays are prorided, one for each outgoing trunk. and these chain relays are connected, one to each oi the release trunk conductors. It follows that as fast as the trunk lines become busy the chain relays will he cnergizeril, and when all of the trunk lines are busy all of the chain relays will be energized. with the re sult that round is placed on the conductor thus closing an energizing circuit for the relay 83. Upon energizing, relay S3 closes at armature 89 a circuit for relay 85, which upon attracting its armaturcs connects ground to the conductor 98. The release trunl: conductor il of the trunk line which tern'iinates in the secondary line switch D normally connected to the conductor 93, and it the trunk line is not otherwise engaged it is made busy anyway. Relay 85 may he a large mu tiple contact. or gang relay, or a number of relays connected in series or parallel may be used; at any rate, at the same time that conductor 93 is grounded similar conductors exteuding'to all of the remaining seccudary line switches in the same group are grounded also. thus nrahing all ot the trunk lines incoming to the group in question busy.

As a. further result of the energization of relay 83, upon all of the trunk lines becoming busy the conductor 92 is grounded momentarily tor the purpose of operating the master switches in all of the primary groups irozn which trunk lines lead to the particular secondary group in question. The slow acting relay is normally energized, but when relay 83 attracts its armature the circuit of relav Q1 is broken. Relay 8%, how- ()K crcr, does not retract its armature immediately and in the meantime relay 3 places ground on conductor 92 by means of its an mature S8. The grounding of conductor 92 Ill) up. ....H

fa l

4;, oaaaerz.

is: only l'IHHHQHl-Rl) as ground is taken oil at ari'l'ialuie Elli as Soon as relay 84-. t'alls bark. When eoudurlor S 2 is grounded in this mannor a circuit in eonuileted through contacts ol her ll? lo the eonunou emulur'lor 2th and lhenoe h way of the normally elosofl swilrh t eontart ot hey K eondmlor Eli, and the winding: of relay 1.012 lo lsallery. ll ron energizing. relay i013 oslahlislun a lorlnngg' rirunit For ilaelii al ila arinaluro lFlll and at ita aruialure lill ('lUFV? the rirruil of line look inn relay llll. l pou energizing, relay .l l eloaea a eirruil For the open main. relay um and at the value linle unlorloi the, master awileli aha l'l nierlianirally so that it in tree lo more to the lel't or in a rounter rloeli-wirle ili eet in uiuler ronlrol ol' the main spring; and ;ernor. zu previously explained. l iuro relay lll'i has luen lurked in energized ooailioih lhia niorenienl will eonlinue until the wipers llli' and loll arrire at llie first loat ronliu'la in their respriairo l anlqa at whirh time the line au'ilrh nhu'ui'era will he opposilo the l'irr l lrunh. The arm lei I2 now l'lOFOS roiuart springs iii-l. and 12: lhuu roinnletingzi a circuit i'or relay lO-l. ll hen relay to lrarla il arnrilure, in opera. ed position hehind the loolh n spring lit? so that the relay remain. in (111-- eraleil poailion a'ller its euerlif i iinn rirruil is lHUli-lll. The ener g'izializui of relay llll lireahs lho locking eireuit of relay h n inilline' this relay to tall haelc. and also closes a new lira-nit l'or lhe lorhing; relax 10L in order lhat lhis laller 'roaty may he held up ailior llie relay 10? has deeiuu-n izzed. in additioiu relay llll; lo us a eirruil tor the so enoid lOti. l oon the enernization of the aolenoid. the rnaaler awileli shaft is rotated in, a eonlinuoua moieinenl in a clock-wise ilireelion until all ol lhe nlungz ers are lined up oprun-ile the tenth trunk. wipers ll)? and ll? heine' reatored to the position FllOWD in the drawing. The arm 1533 now d sen rages s n-inn; tea from the aanalure of relay 'lO-il and this arnialure is role: Sell to break the e rruil o l the solenoid 'ltlll and the cireuil of relay llll. Relay: 10! now deenerp'izea and lorhs the shall in position opposite tho enlzh trunk. 'lhere is nothing}; parlioularly new in most of those oi u raliona al the iuaaler switch and the have heen dearrihed merely in order to llllliil clear toe purpofie of placing mo uwular ground on ronduetrw .lrB under the i-ir uiusiam-e" .llilltlll. Obviously when lhe nriinarr nirr swilenea are operated. any line switch nhtnggorsa whieh liar/pen lo he ilii' engaged tron! lhe ehal'l are pulted up" and aragrain hroug'ht under lhe eonlrol ot the master switch so that there will he no danger oi' an ol then! lieino plunged in on hurry trunk liner".

:inolher Feature 'a a provision 'lor n'nihiiu; all the incoming trunks at a group ot SOC- ondary line swilrhes huay and for operating lho arumlure is lor the primary nuuiter switches, as ClGFSClllJQtl in the 'loregroing, in use the main Fuse which supplies operating hattery lo the line switches ol. the group should lilow. Considurine; this l'eatui'e a little further the rel-- ereiu e rharzmler ill? indieatea the main l'uuo whirh supplies operating current to all. of the .we-i-oiulary line airilrhes which are as sorialod with the lino awilrh ill in the same ;'l' lll7. ll will he understood. of L'Ull! 'E-'(!,l1lliil1 in przu'lii-o relay no and the stepping uuigrnet i13 instead of l ving ((Fl!l|(!ll? l diroel lo the im rronndeil polo ol" the liallery are. eonneeled lherelo h war ot the lose an. as; are the relays ll l au-rl at. No\\, il this; fuse should hlor; 'l'or any reason all ol' the new (aulary line SWllii'llQS in the group will at once heuoine inoperative. and it will elearly heroine neeessarv to render all the incoming; lruulc lines huay, and also to operate all the primary uiaater switches in order to pieli up our line awiteh llllllh' l'il'S whirli may happen to he alandiuo' on trunk lines extending to lhe secondary group in question. This is :uttdllPllFlllll a2 :l'ollowa: ll hen the time blows, relay 2-:51- will deenere'ize and by means of its arniatin'es $16 and 90 circuits are closed simultaneously for energizing; relay S5, and tor phu-ingr a ground on conductor 92. Re lay S5 energized, gg'roui'idingz conductor llll arul eiuiila r eoruiluetors in order to maho the .i morning: lrunk lines hussy while all; arinalure 91 this relay removeo ground from rondurlor (loiulnelor $352 is thus grounded nunuentaril a. in the e: 5e )l'0\"l()1'lSly explaiued, and hi lhe rouseunent locking no oil? relay .1052 and similar relays associated with the other primary master swilehes, all ot these master Switches are one 'aleil as and for the purpose already set 'lorth.

The he; l anal l7, loo-ether with a 'lllll" ralily oi. olhor similar keys may he loraled on a test panel in some eonrenient place in the switch roonu and are novided tor the purpose of enahling an attendant or switch hoard main lo lorale trouble on the cominon eondurlor 200 and its Ya ions hranehes. Thore may he as high as halt a dozen groups ot secondary line switehea, and twenty-five or more groups o'l primary lino switches, from whirh it will he clear that lhere may he nix hranehes eorresnoiuliiur to lu'anehes l2 and 9 -3, and twoiitv-lire or more hranrhes eorrespoiulingr to the hraneh 94- and it will he amireeialeil lhat ii! any one o'l't those branches should lieuouie permanent y grounded in some wtho ronsequenres would he exlreniely serious. The teal heya are provided therolore, lo ena le lhe attendant to quickly lUl'il-l't! surh trouhle it it should oorur. which is tlli't'fiilll llHlltal hr operating; the he sueeewsirelr and ohaerring the sound ot' the humor 93. is soon as any particular he is' operated the lnraneh associated with such lugs diseoni'ieeted from the common conductor 200, and the latter conductor is connected with the buzzer. If it should happen that the branch associated with the operated key is the one which is grounded, the common conductor will now be clear and the buzzer will be silent. However, if the wrong key has been operated, the common conductor will still have a ground on it and the buzzer will operate, indicating to the attendant that the trouble has not yet been located. Obviously if the keys are operated in quick succession a few seconds only will be required to locate the particular branch which is in trouble.

The key K is known as an emergency pick up key, and may be mounted on the test panel with the keys just described. It is useful to the attendant for trying the operation of the primary master switches, and may also be used in case the re ular cir cuits for automatically operating these master switches should get out of order. The exchange is always equipped with various supervisory signals and alarms which indicate the blowing of fuses, or the busy con dition of trunk groups, etc. The attendant is thus, at all times, aware of the condition of the equipment and in case some of the automatic devices fail to perform their functions he may accomplish the desired results by means of the emergency key.

As stated hereinbefore, a number of meters are provided for recording certain events occurring during the operation of the system, and these meters and their functions will now be explained. The peg count meter M will be considered first. It will be observed that the common conductor 44:, which supplies battery to the pull in windings of all the primary line switches of the group which includes the line switch C, includes the winding of the low resistance relay 112, and it will be clear, therefore, that whenever one of these line switches is operated relay 112 will be operated also.

Each time relay 112 operates it closes a circuit for the meter IV and this meter, therefore, will register the total number of calls outgoing from the group in question.

The meter M is the all trunk busy meter, which functions as follows: In case all of the trunks outgoing from the primary line switch group are busy all of the test contacts in the bank associated with test wiper 107 will be grounded, and the starting relay 100 will remain energized as long as this condition continues. lVhen relay 100 is energized it closes a circuit for the locking relay 101 in order to initiate the operation of the master switch, as explained before, and it also closes a parallel circuit through the relay 111. This latter relay is a special relay constructed as shown in the pending application of John Erickson, Serial No. 348,548, filed December 31, 1919. When its armature is attracted a circuit is come pleted for meter M, but at the same time a pendulum is set in motion which in vibrating rapidly interrupts the circuit of the said meter and thus for a certain definite time prevents it from operating. A relay of this type is used so that in the ordinary operation of the system the meter M will not be caused to register even though relay 111 is momentarily energized from time to time. The time interval of the pendulum controlled by relay 111 is of such duration that the master switch can operate in two complete cycles, and thus the meter will not be operated during the ordinary operation of the master switch in stepping from trunk to trunk, or even when the master switch moves the plungers from the first to the tenth trunk, while if all the trunks are busy and the master switch is in continuous operation, the meter will be caused to register.

The remaining meter M is provided for registering calls which fail because no outgoing trnnks are available. It will be observed that the common conductor 46, to which the lower side of each subscribers line is connected, extends to the winding of relay 110 to ground. Relay 110, however, is normally short circuited at the armature of the starting relay 100, and it follows, therefore, that in the ordinary operation of the system when a subscriber removes his receiver and operates his line relay, such as the line relay 16, relay 110 will not be energized. In case all the trunks are busy, however, the starting relay 100 will be held up continuously and relay 110 will not be short circuited. Under these circumstances, if another subscriber should attempt to make a call, when he removes his receiver, relay 110 will be energized. This relay is of very high resistance and prevents the line relay of the calling line from pulling up at this time. Upon energizing, relay 110 closes a circuit for the relay 109, which is a relay equipped with a pendulum device the same as relay 111. If the starting relay 100 does not deenergize before the expiration of a definite time interval, relay 109 will permanently close the circuit of the meter l\ 3 in an obviouamanner and the said meter will be caused to register.

When the circuits are connected as shown in the drawing, if the starting relay falls back before meter M has operated in the case just described, relay 110 will immediately be short circuited again and it will fall back to break the circuit of relay 109 which in turn will restoreits armature and prevent the meter M from being operated, as explained in the preceding paragraph; but if the jumper containing terminals 130 and 131 is cut the operation will be different. With this arrangement of the wiring the lamps, such as the lamp L at the wire chiefs,

or complaint operators desk, are preferably inserted in their jacks and when the relay 110 operates in response to a call, during a time when all the trunks are busy, will open the short circuit around its winding and its upper armature so that it will not be restored until the calling subscriber replaces the receiver, even though in the meanwhile one or more of the outgoing trunl; lines should become idle. At the same time, relay l'lO closes a circuit for the lamp L and thus notifies the wire chief that some subscriber .in the group associated with said lamp is trying to call under circumstances which are bound to render his attempt unsuccessful, and the wire chiei will, therefore, plug into the jack J with a cord circuit, with which his position is provided, and will talk to the subscriber and advise him to try the number over a 'ain alter a "few minutes. Relay 110 it inay be mentioned is made slow to energize by providing it with a copper slug on its armature end. This is done so that it someone should attempt to call during a momentary energization of the starting ,relay 100, while the master switch is stepping from trunk to trunk, the relay 110 will not enerprize and will not become locked up under these circumstances.

Having described my invention, what 1 consider to be new and desire to have protected by Letters Patent will be pointed out in the appended claims.

\Vhat 1 claim as my invention is:

l. In a telephone system subscribe i lines, trunk; lines, groups at primary line switches and a group of rotary secondary line switches for extending calling lines to idle trunk lines, there being a secondary trunk line extending; from each primary group to One of said secondary line switches, chain relays associated with said first named trunk lines, each relay being energized when the associated trunk becomes busy, a relay normally deenersrized, means controlled by said chain relays when all the associated trunks become busy for "arising said normally do energized relay to energize, and means con trolled by the energization of said relay for making all or said secondary trunk lines busy, said means including circuits extending through contacts on relays in each ot the secondary line switches.

2. In a telephone system,subscriberslines, trunk lines, groups of primary line switches and a group of secondary line switches for extending callinglines to idle trunk lines, there being a secondary trunk line extending from each primary group to one of said secondary line switches, a main fuse supplying ope 'ating battery to the secondary line switches oi: said group, a. relay normally energized by current supplied through said fuse, and circuit connections for making all of said secondary line switches busy in case menace said relay is deenergized by the blowing of said :tuse.

In a telephone system,subscriberslines, trunk lines, groups of primary line switches and a group of secondary line switches for extending calling lines to idle trunk lines, there being a secondary trunk line extending from each primary group to one of said secondary line switches, a master switch in each primary group controlling the associated primary line switches, a main iusc supplying operating current to the secondary lino switches oil said group, a relay normally onergized by current supplied lhrrjuurh said fuse, and circuit connections for operating all of said master switches in case said relay deenergized by the blowing; of said tuse.

4;. In a telephone system,subscribers lines, trunk lines, groups of primary line switches and groups oi. secmulary line switches for extending calling lines to idle trunk lines, a master switch for Ulltl'tllllllg each group oi prin'iary lino switches, a common conductor branching at one end to each primary grou p and atthe olhe r end to each seccmdary group,

means associat d with each :uucondary group for placiiua a potential on the branch cxtending thereto in case all the trunk lines accessible to the switches in such group become busy, and means controlled over each primary branch For operating]; the associated master switch.

In a teleplumc system, subscriberslines, trunk lines 'roups of primary line switches and groups oi a'ol'ulary line switches for extending calling lines to idle trunl; lines, a master switch. for controlling each group ol primary line switches, a common cmuluctcr branching at one and lo each primary grou p and at the other end. to each eecoiular roup, means associated with each secondary group tor placing" a potential on the branch extend-- inp; thereto in case all the trunk lines accessible to the switches in such group become busy, means controlleul over each primary branch tor operating}; the aiemciated n'iaster switch, a signal, and test he 4 in the several branches tor use in coniimctiou with said signal to locate an accidental potential on any liu'anch.

(3. In a telephone system, subscribers lines, trunk lines, groups of primary line switches and groups oi secondary line switches for extending; calling lines to idle trunk lines, a master switch for (uimtrolliiur each group of primary line switches. a common coutluctor branching; at one end to each primary group and at the other end to each secondary group, means associated with each secondary group for placing a potential on the branch ex tending: thereto in case all the trunk lines accessible to the switches in such group become busy, means controlled over each. primary branch for operating the associated master switch, and an emergency lccy manu- Hill ill)

llii

ally operable to place a potential on said common conductor, whereby said master switches and the operating circuits therefor may be tested by an attendant.

7. In a telephone system, a group of subscribers lines, a group of trunk lines, individual line switches for extending calling subscribers lines to idle trunk lines, a relay, circuit'connections for operating said relay in case a line should attempt to call while all oi said trunk lines are busy, a manual jack common to all of said lines, and a signal associated with said jack and controlled by said relay. 8. In a telephone system, a group of sub scribers lines, a group of trunk lines, individual line switches for extending calling subscribers" lines to idle trunk lines, means dependent on the busy condition of all trunk lines for preventing the operation of any line switches, and means whereby in case a call is attempted while all the trunks are busy the possibility of operating any of said line switches is made dependent on, the continuation of the attempted call.

9. In a telephone system. a roup of sub scribers lines, a group of tru lr lines. line switches individual to said subscribers lines arranged to establish connections between said subscri ers lines and said trunk lines, and means dependent on the busy condition oi all the said trunk lines when a call is initially attempted to prevent the subsequent operation of any of said lineswitches.

10. In a telephone system, a group of subscribers lines, a group of trunk lines, line switches individual to said subscribers lines arranged to establish connections between said subscribers lines and said trunk lines, and means initially operated when a call. is attempted and all trunks are busy but maintained operated by a line attempting to maize a call, said means when operated preventing the operation of any of said line switches.

11. In a telephone system, subscribers lines, a group ot plunger type primary line switches, trunk lines accessible to said switches and leading to groups of rotary secondary line switches, different trunk lines accessible to each of the groups of secondary switches, a master switch arranged to move the plungers of said primary switches opposite a different trunk line when the trunk line opposite which they rest is connected to give a busy indication, a relay operated when all the trunks accessible to a secondary switch are busy, and a circuit including two branches, one of which extents through a front contact on said secondary switch and the other branch through a front contact on said relay and a back contact on said secondary switch, for giving a busy indication.

12. In a telephone system, a primary line switch, a secondary line switch accessible to said primary switch, a source of E. M. I. .t'or operating said secondary line s itch, and means arranged to cause said secondary switch to give a busy indication to said primary switch it the source of E. M. F. ceases to supply E. M. F. to said secondary switch.

13. In a telephone system, a group of switches, a second group of switches, trunks accessible to the second group oi switches, each of the switches in the second group being accessible to each of the switches in the lirst group, means for indicating said .tirst mentioned switches a busy condition of said switches of the second group, connections including a battery for operating the second group of switches, and means common to all the switches in the secondary group for giving the busy indication if the ba tery fails to supply E. I*. to the switches.

1st. In a telephone system, a group of switches, a second group 01" switches, trunks acces..,,ble to the cond group o't switches, each of the switches in the second group being accessible to each of the switches in the first group means for indicating to said first mentioned switches a busy condition of said. switches of the second group, connections including a battery for operating the second group of switches, and means common to all the switches in the secondary group for giving the busy indication if the battery fails to supply E. M. F. to the switches, and also when all the trunks accessible to said secondary switch are busy.

15. In a telephone system, wherein a group of switches having access to a munber of trunks are themselves accessible to another switch, and each of the switches of the group is provided with means for indicating a busy condition to said other switch, and a source of E. M. F. is provided for operating the group of switches, means common to the group oi switches arranged to give the busy indication if the E. M. I ceases to be supplied.

16. In a telephone system, wherein a group oi switches having access to a number of trunks are themselves accessible to another switch, and each of the switches of the group is provided with means tor indicating a busy condition to said other switch. and a source of E. M. I is provided for operating the group of switches, means including a relay common to the group of switches and normally energized from said source of E. M. F. arranged to give the busy indication when deenergized, and means for deenergizing said relay when all the trunks accessible to the group of switches are busy.

17. In a telephone system, a relay nor mally shunted to prevent its operation, a group of subscribers lines, a group of trunk lines, line switches individual to said subscribers lines arranged to establish conneclUS lit)

tions between said subscribers lines and said trunk lines only when the shunt is unbroken, means operated when all the trunks are busy to break the shunt around the relay, a connection whereby the relay is operatinl responsive to the initiation ol a call while the shunt is broken, and means controlled by the relay for maintaining the shunt broken independent oi? the busy or idle condition ot the trunks.

ltl. In a l'PlQPilOHO system, a relay normally shunted to prevent its operation. a group of suhF rihers" lines, a group oi trunk lines, line switches individual to said subs-werihers lines arrainred to estahlish connections between sau merrihers lines and said trunk lines only when the shunt is unhrolmn, a chain of relays one For each trunk, connections :tor eperi'iting each relay when its trunk becomes busy, means operated when all of the chain relays have oi ierated E'or breaking the shunt around the first n1entioned relay, eonnectimis \i'herelr said lh'st relay is operated responeire to the initia ion of a call while the shunt is broken, and means controlled by said tirst relay 'l'or maintaining the shunt broken independent of the busy or idle condition of the trunks.

19. In a telephone system, a line, a group of trunks accessible thereto, means responsire to a c2 ll on said line for connecting the same with an idle trunk of said group, means for semling directing em'rent impulses through said line and the connected trunk, and means operated it an attempt is made to make a call when all of the trunks are bus) to prevent the seizure of a trunk even after a trunl: becomes available, whereby said in1- pulses are rendered .inellfeetiye tor directive p u rposes.

so. in a telephone system, a line, a group ol? trunks accessible thereto, means for sendin; current impulses through the line, said impulses being normally elleetive to establish desired connections from the line, and means automatically operated when all of the trunks are busy to render the impulses inetl'ecliye to extend, comumtions even after atrunk becomes idle.

t i n'ned by me at Chicago, look .lounty, lllinoia, this 4th day of April, 1925.

'JTALHO'I? (.l. MARTIN. 

